heterodyne: Add recipe parameters to control trimming of noisy spectral ends.
heterodyne: Just apply offset to the next subband.

No longer using MATHS so there is now no need to copy the units and
restore them.
heterodyne: Indent log commentary.
heterodyne: Allow for the subbands not to be in increasing WCS bounds order.

The TIMESORT step puts them in chronological order, which is not
necessarily adjacent sections of the spectrum.
heterodyne: Remove diagnostic.